The invention relates to broadband transmission of information. More specifically, the invention relates to time synchronization of a received OFDM signal with a reference.
Orthogonal Frequency Division Multiplexing (OFDM) is a spread spectrum technology wherein the available bandwidth is subdivided into a number of discrete channels or subcarriers that are overlapping and orthogonal to each other. Each channel has a well defined frequency. Data are transmitted in the form of symbols that encompass the subcarrier frequencies. The amplitude and/or phase of the subcarriers represents the encoded information. Before a received OFDM symbol can be decoded, frequency and time synchronization of the signal have to be determined. This process is referred to as synchronization.
In OFDM transmission across power lines, the frequencies are low and crystals that provide good frequency control and stability are readily available. Frequency synchronization, however, may become more difficult at higher frequencies, e.g., in radio transmission.
Time synchronization of the received OFDM signals is more difficult to achieve, in particular in the presence of narrow band interference in the OFDM channels.
OFDM signals are typically synchronized using a time domain correlator, i.e. a correlator operating in the time domain. An OFDM transmitter and a corresponding OFDM receiver typically operate at identical and stable subcarrier frequencies, but the clock cycle defining the symbol times in the transmitter and receiver may be offset with respect to each other by a certain symbol time offset. Time synchronization between the transmitter and receiver clock cycles is accomplished by determining this offset. Typically, the receiver monitors the OFDM transmission channels for a predetermined synchronization symbol. Once the offset is established, all subsequently received symbols are shifted by the amount of the so determined offset and thereby aligned.
In one approach known in the art, the time domain correlator computes cross-correlation values between a stored temporal symbol pattern and a received temporal symbol by shifting the stored symbol pattern in small time steps relative to the received symbol until a maximum correlation value is found. The subsequently received OFDM symbols are then shifted in time by the total time shift that produced the initial alignment between the received symbol and the stored or reference symbol. This method, however, requires computation of a large number of digitized values of the received symbol. Correlation is also difficult to establish if the received signal is corrupted by narrowband interference.
In another approach known in the art, a received symbol is incrementally time-shifted in small time steps over the time duration of the reference symbol. At each time shift, the received symbol is Fourier-transformed (FFT) into the frequency domain. A cross-correlation is carried out in the frequency domain, wherein a correlation maximum is obtained when the FFT operation is performed at exactly the correct time shift. This method has the disadvantage that a large number of FFT operations have to be performed before phase correlation (time synchronization) can be established.